The blunder happened on May 11 at the RAC Arena in Perth ... just after Cherneka Johnson and Nina Hughes went toe-to-toe against each other for 10 rounds with the WBA bantamweight belt on the line.

Hennessey read the three judges' scorecards ... but, he said Hughes won -- when the real winner was actually Cherneka.

Hughes celebrated the victory with her team ... but Hennessey -- who's known for his catchphrase, "Would Somebody, Somewhere, Make Some Noise!" -- hopped back on the mic to re-read the scorecards ... and reverse the call.

"For your winner by majority decision," he yelled, "fighting out of the blue corner -- Cherneka Johnson!"

Hennessey was immediately hit with criticism ... with the fight's play-by-play announcer, Joe Tessitore, blasting him on-air. Later, fans on social media came after him too ... and after the backlash grew so loud, Hennessey said Wednesday he simply could not continue in his role.

"I love all the support from everyone," he said in a post on his Facebook page. "Thank you all for the kind words. Unfortunately, the worldwide backlash is absolutely incredible and it's affecting my mental health to a degree where I will have 1 more show ever."

"I love and will keep in touch with all my friends from around the world," the 56-year-old continued. "Thank you.. No longer the world's punching bag. I'm out."

Hennessey started his ring-announcing career at the Kings of Oceania kickboxing event ... and got his big moment in 2009 at the David Tua vs. Shane Cameron fight, the biggest PPV in New Zealand at the time.

After that, he became the main announcer for Duco Events and called several big-name fights, including ones involving Manny Pacquiao and Andy Ruiz.